Field of Invention
This invention is an economical method for recovering phosphate as a product from liquid streams through the use of autotrophic organisms that increase the pH and reduce the alkalinity of the liquid stream thereby reducing the need to purchase chemicals that increase the pH or reduce the alkalinity for the rapid production of phosphate precipitates.
Background
Phosphorus is an essential plant nutrient required for food production. The world has a limited supply of phosphorus to meet those needs. Unfortunately, most of the mined phosphorus is discharged or lost to the environment during food production and consumption. The discharge of phosphorus from agricultural drainage and waste treatment facilities also creates significant environmental problems such as stream, lake or estuary eutrophication resulting in reduced dissolved oxygen levels needed to support aquatic life. Phosphate also creates waste treatment difficulties by forming uncontrolled precipitates that clog pipes and hinder waste processing. As a result there is an intense interest in developing processes to recover phosphorus for reuse and to control unwanted precipitate formation. Phosphorus can be recovered from waterways, within waste treatment facilities, or the downstream anaerobic waste stabilization processes commonly used in municipal or agricultural waste processing. Anaerobic treatment processes are designed to reduce the volume of the solids while converting those solids to soluble inorganic and gaseous products. Anaerobic digestion causes the destruction of solids and the generation of methane, carbon dioxide, and hydrogen sulfide gases along with soluble inorganic products such as ammonia, phosphorus, magnesium, and calcium. The ammonia and phosphorus concentrations found in an anaerobic digester's effluent (digestate) are typically high. Ammonia concentrations of 500 to 3,500 mg/L and phosphorus concentrations of 50 to 500 mg/L are not uncommon. On the other hand, the phosphorus concentration in streams and waterways should be less than 0.1 mg/L.
To be recovered, orthophosphate (dissolved phosphorus) is converted to the particulate (solid) form and removed as a particulate by sedimentation, screening, filtration, or as a chemical precipitate that may or may not be crystallized. Phosphates that are associated with particulate matter can be screened through a series of fine screens. There are three basic phosphorus removal processes used in wastewater treatment: chemical, biological, and nano. Nano processes are processes that remove very fine particular matter through processes such as fine screening or membrane filtration. Biological phosphorus removal consists of accumulating dissolved phosphorus within phosphate accumulating organisms (PAO's) and exporting that particulate phosphorus in the biological waste stream (biosolids). Chemical processes convert phosphorus to a chemical species by adding a metal salt such as iron, aluminum, magnesium, calcium, lime or a combination of those chemicals. The efficiency of chemical phosphorus removal is dependent on two factors: the chemical equilibrium between the phosphorus liquid and solid phases and the efficiency of the solids removal process. Typically, the latter process controls the removal efficiency while the chemical use controls the cost.
Removal of phosphate by chemical addition is attractive for its simplicity of operation and ease of implementation. The commonly used chemicals are aluminum [Al(III)], ferric [Fe(III)], calcium [Ca(II)], Lime (CaO) or Magnesium (MgO, MgOH, MgCl2]. On the other hand those chemicals may increase the alkalinity thereby increasing the time required for precipitation of various phosphate compounds. Chemical treatment provides an opportunity for nutrient recovery of both phosphorus and nitrogen. In particular, controlled struvite precipitation processes, such as DHV's Crystalactor® (Netherlands), Paques' PHOSPAQ® (Netherlands), and Ostara Nutrient Recovery Technologies' PEARL™ process, are emerging processes that recover both nitrogen and phosphorus. Struvite or magnesium ammonium phosphate (MAP) precipitation is effective for phosphorus control, but cannot substantially reduce ammonia concentrations in the digestate. Available phosphate generally limits struvite ammonia recovery to less than 20%. However, struvite recovery is one of the few if not the only process that recovers some nitrogen and phosphorus. This invention does both. It can remove substantially all of the nitrogen and phosphate as any of a variety of nitrogen compounds and calcium phosphate, potassium ammonium phosphate (KAP), or magnesium ammonium phosphate (MAP), also known as struvite.
It is well known that aqueous phosphate can be very effectively precipitated with an excess of Ca++ ions as calcium phosphate under conditions of high pH and low bicarbonate alkalinity. The addition of hydrated lime [Ca(OH)2] is very effective in achieving both conditions since it provides the calcium required while increasing the pH. Lime, however, is a cumbersome chemical to use. Similarly, magnesium oxide (MgO) or magnesium hydroxide (MgOH), with or without NaOH, can provide the magnesium necessary while increasing the pH required for struvite or MAP recovery. Less costly chemicals, such as calcium chloride or magnesium chloride, can be used if the pH did not have to be raised to levels of 8 to 10 through the use of caustic chemicals such as NaOH, and MgOH. Less costly chemicals such as CaCl2 (common road salt) or MgCl2 could be used if the pH increase and alkalinity reduction were performed by an alternative process.
Formation of calcium phosphate requires low bicarbonate alkalinity. Ferguson1 found that the rate of calcium phosphate production was primarily dependent on low alkalinity. For example, in one of his demonstrations he showed that the time required to remove 0.05 mM of phosphate at an alkalinity of 3.5 mM was 100 hours, whereas the time required to remove the same 0.05 mM of phosphate at an alkalinity of 0.5 mM was less than 10 hours. Alkalinity had a profound impact on the rate of removal, the required size of the reactors, and the efficiency of the process. Angel2 had a similar experience. He successfully demonstrated at pilot scale the removal of 98% of sewage phosphate as Ca2(PO4)2. However, when he applied the technology at full scale it failed economically due to the “real world” higher alkalinities. 1 Jenkins, Eastman, and Ferguson, Calcium Phosphate Precipitation at Slightly Alkaline pH Values, Journal WPCF Vol. 45, No. 4, April 1973 page 623.2 R. Angel, Removal of Phosphate from Sewage as Amorphous Calcium Phosphate, Environment Technology, Vol 30, pg 709-720, March 1999
Previous research has established that economical chemical removal and recovery of MAP or calcium phosphate requires high pH, low alkalinity, and highly soluble chemicals to provide magnesium or calcium ions. This invention provides those required conditions.
Burke3 received a patent on a process to recover ammonia nitrogen from liquid waste streams without the use of chemicals. The process is an economical method for recovering nitrogen from liquid waste using autotrophic microorganisms in a photobioreactor without chemical additives. The organisms consumed CO2 and bicarbonate thereby reducing alkalinity while increasing the pH. The liquid containing phosphate is introduced into a culture of autotrophic microorganisms in the presence of natural or artificial light, thereby producing a liquid effluent with an elevated pH and a substantially reduced bicarbonate alkalinity. While operating Burke's nitrogen removal process to recover ammonia from a highly concentrated digestate, it was observed that a precipitate began to accumulate in the photobioreactor. While demonstrating the process, it was also observed that the reactor was substantially devoid of calcium and that the alkalinity was very low due to the consumption of bicarbonate by the phototrophic organisms being grown in the photobioreactor. Further investigation established that the precipitate was amorphous calcium phosphate. A number of experiments were subsequently conducted to further remove phosphate by adding either calcium chloride or magnesium chloride to the effluent. Rapid (several minutes) precipitation and settling of calcium phosphate occurred upon the addition of small quantities of calcium chloride. Similarly a slower forming precipitate was produced upon the addition of magnesium chloride. It was clear that another method of economically removing and recovering phosphate from liquid streams had been discovered. The method does not require the addition of pH-increasing and bicarbonate-reducing chemicals. Simple inexpensive chemicals, such as ammonium chloride or magnesium chloride, could be used to supply the required Ca++ or Mg++ ions. 3 Dennis A. Burke, U.S. Pat. No. 8,637,304, Jan. 28, 2014.